Aberrant G protein-coupled receptor (GPCR) signaling causes many human diseases including cancer. Regulator of G-protein signaling (RGS) proteins inhibit GPCR signaling. The long-term goal of this application is to elucidate the roles of RGS proteins in tumorigenesis. Prostate cancer is the most common cancer in men and acquisition of androgen independence by prostate cancer is the key problem of prostate cancer progression. Several lines of evidence suggest that androgen-independent activation of androgen receptor (AR) is one of underlying mechanisms. Recent studies demonstrate that hormones acting through G protein-coupled receptors (GPCRs) cause androgen-independent AR activation. We recently reported that RGS2, a member of RGS superfamily, selectively inhibits androgen-independent AR activation by GPCR signaling in prostate cancer cells. Interestingly, RGS2 protein expression was inversely correlated with the androgen-independent cell proliferation in established prostate cancer cell lines. In addition, human prostate cancer specimens had significantly lower levels of RGS2 compared to normal prostate tissues. Thus, we hypothesize that RGS2 functions as a novel regulator of AR signaling and its repression is an important step in prostate tumorigenesis and progression. Specific Aims: 1. To assess the biological importance of RGS2 repression in prostate cancer cells. 2. To determine the molecular mechanism underlying RGS2-mediated inhibition of androgen-independent prostate cancer growth. 3. To elucidate the mechanism underlying RGS2 repression in prostate cancer. Research Design and Methods: We will first examine the effects of altering RGS2 expression on the androgen-independent growth of prostate cancer cells in culture and in castrated athymic nude mice. We will then pinpoint the amino acids of RGS2 critical for its selective inhibition of androgen-independent AR activation and cell growth in prostate cancer cells and identify the AR domains responsible for its regulation by RGS2. Finally, we will combine methylation-specific PCR and immunohistochemistry analysis to investigate whether methylation of the RGS2 gene is responsible for RGS2 repression in prostate cancer. The feasibility of using RGS2 expression and/or RGS2 methylation status as a novel marker for diagnosis and prognosis of prostate cancer will then be evaluated. In addition to methylation, we will also explore whether genetic alterations or histone modifications contribute to RGS2 repression in prostate cancer. Information on the biological importance of RGS2 in the regulation of prostate cancer progression is directly relevant to understanding the biology of prostate cancer. Completion of the proposed studies will not only provide a potential diagnostic and prognosis marker for prostate cancer progression but will also allow the development of novel drugs for the treatment of prostate cancers.